There are many options not just asus, even linksys, netgear, tp link and so on. If you want MU-MIMO go for the qualcomm based ones. The additional 4 ports on the AC88U are bottlenecked (sure people argue its the same as attaching a 5 port switch but its in the same device, no reason to have bottlenecks or to not make use of the additional RGMII available on the switch chip. The mikrotik RB3000 has 2 5 port switches, each having 2Gb/s to CPU). If you want a router for the ports, go with mikrotik. CCRs can give you 12 gigabit ethernet ports, the RB1100AHx2 can give you 13 and even the RB3000 has 10 + SFP. For the switch chips on mikrotik with 1Gb/s to CPU you can connect the switches together externally for even more links.
Hence why my advice against the AC88U, just not worth the cost of the extra 4 ports unless you plan to attach 1Gb/s device + 3 100Mb/s device for example to the extra ports. Network printers, raspberry pis are example devices with only 100Mb/s ethernet.
Routers are routers, your gameplay isnt going to be improve just by going for asus, what asus calls by gaming is simply the look and a few unnecessary features (like WTFast vpn). a good router like mikrotik can do so much more as evident by netduma which is basically mikrotik with a friendlier interface. A lot of people complain that ASUS routers QoS profiles gives bad latency when tested but it could be that the QoS is set to give games the priority they need with everything else second. Same can be achieved on mikrotik and other routers too. For consumer routers you'd need to be able to get on the underlaying OS to configure some complicated QoS.
So pick between these variants for not just asus, other brands. Look at both the hardware and firmware if it fits what you want (remember there is no such thing as a gaming router as far as routing goes, only router related features like QoS type available, port forwarding, etc).
The AC1900, the AC3200 (2x AC1900 5Ghz), AC3100 (MU-MIMO), AC2600 (qualcomm MU-MIMO), AC3200 (other MU-MIMO with 2x 5Ghz radios such as the linksys wrt).